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FILE - In this March 6, 2013 file photo, a portion of the Hanford Nuclear Reservation is seen from the air near Richland, Wash. A Department of Energy contractor is checking after more waste leaked from the inner tank of ... more >

SPOKANE, Wash. (AP) - Hanford Nuclear Reservation officials prepared Tuesday to pump thousands of gallons of leaked radioactive waste back into a 46-year-old storage tank that contains toxic leftovers from the production of plutonium for nuclear weapons in Washington state.

Government officials said none of the radioactive waste appears to have been released into the environment, and there is no known danger to the public.

“The good news is right now we have no indications that waste has reached the environment,” said the U.S. Department of Energy, which owns the Hanford site. “We continue to monitor it and have leak detectors in the area.”

More than 3,000 gallons of radioactive waste leaked Sunday from the inner tank of a giant double-walled tank known as AY-102 into the space between the two walls.

Officials for federal contractor Washington River Protection Solutions were getting ready Tuesday to begin pumping the leaked waste back into the main tank, spokesman Jerry Holloway said.

Hanford for decades made plutonium for nuclear weapons, and now contains millions of gallons of radioactive waste stored in 177 underground tanks. The government is spending some $2 billion a year to clean up the sprawling site, a process that is expected to take decades.

The leak on Sunday occurred as workers were pumping out the contents of the million-gallon tank. Only about 45,000 gallons remained in the main tank, much of it in the form of a thick sludge.

Holloway said one theory for the leak is that efforts to remove the sludge, which involves shooting liquid through a nozzle, exposed a hole in the inner wall of the tank that allowed more than 3,000 gallons of waste to pour into the space between the two walls, which is called the annulus.

Pumping equipment had already been installed in the annulus in anticipation of such a leak, Holloway said.

In past years, about 70 gallons of waste had leaked into the annulus and dried in three spots. That is what precipitated the pumping of tank AY-102 in the first place.

Hanford contains 28 double-walled tanks and 149 single-walled tanks, some of which date back to its origins in World War II. Tank AY-102 went into service in 1970, and is Hanford’s oldest double-walled tank. The first leak into the tank’s annulus was confirmed in 2012, Holloway said.

The Energy Department said it is unclear if the exact cause of this leak will ever be determined, as it is too dangerous to actually send workers into a tank.

Hanford critics decried the leak.

“These tanks were not designed to hold waste for decades,” said Brett VandenHeuvel, executive director of Columbia Riverkeeper. “It’s past time to get the waste out of the unsafe tanks.”

Heart of America Northwest, a longtime Hanford watchdog, said the government and its contractors have been slow to acknowledge leaks in the tanks and urged the state attorney general to sue the U.S. Department of Energy.

Under terms of a deal with state and federal regulators to clean up Hanford, the Energy Department in early March started to empty an estimated 650,000 gallons of liquid waste that sat on top of about 150,000 gallons of sludge in tank AY-102. Officials began the more difficult work of emptying the sludge at the end of March.

The Energy Department said 95 percent of the liquid and sludge waste has been removed.